How to avoid transparent layers being overwritten by other layers

When using PHP's GD library for image processing, processing transparent layers is often a headache. Especially when we use functions like imagecopy or imagecopymerge to overlay multiple layers, the transparent part is easily completely overwritten by other layers, causing the final composite image to lose its transparency effect.

This article will introduce in detail how to use the imagecolorallocatealpha function to reasonably allocate colors with transparency to avoid opaque effects caused by mutual overlay between layers.

1?? What is imagecolorallocatealpha?

imagecolorallocatealpha is a function provided by the GD library to assign colors with alpha (transparency) information in an image. Its function signature is as follows:

 int imagecolorallocatealpha ( resource $image , int $red , int $green , int $blue , int $alpha )

in:

• $image : The image resource created by imagecreatetruecolor() .

• $red , $green , $blue : RGB components of color (0-255).

• $alpha : Transparency (0 is completely opaque, 127 is completely transparent).

By assigning a transparent color to the background or fill color, you can avoid being completely overwritten by other layers when merging layers.

2?? Typical problem: Transparent background is covered

Suppose we have two layers:

• Base image (background image);

• Foreground map (PNG with partially transparent area).

If you directly use imagecopy to overlay the foreground image, the transparent area will often appear as black or overlay the base image, because GD will not handle the alpha channel by default.

3?? Solution: Use imagecolorallocatealpha and alpha blending

Key steps:
1?? Enable alpha blending
Before synthesis, make sure to turn off alpha blending, otherwise GD will ignore the alpha channel.

 imagealphablending($image, false);

2?? Assign transparent colors <br> Use imagecolorallocatealpha to assign a completely transparent color to the background.

 $transparent = imagecolorallocatealpha($image, 0, 0, 0, 127);
imagefill($image, 0, 0, $transparent);

3?? Save alpha channel <br> Remember to save the alpha channel after synthesis, otherwise the output PNG will lose its transparency effect.

 imagesavealpha($image, true);

4?? Complete sample code

Here is a complete PHP example that demonstrates how to avoid overwriting transparent layers:

 <?php
// Create a 400x400 True colored canvas
$canvas = imagecreatetruecolor(400, 400);

// closure alpha blending，To fill the transparent background
imagealphablending($canvas, false);

// Assign a completely transparent color
$transparent = imagecolorallocatealpha($canvas, 0, 0, 0, 127);

// Filling canvas as transparent background
imagefill($canvas, 0, 0, $transparent);

// Loading foreground PNG picture（With transparent area）
$foreground = imagecreatefrompng('https://m66.net/images/foreground.png');

// Overlay the foreground on the canvas
imagecopy($canvas, $foreground, 50, 50, 0, 0, imagesx($foreground), imagesy($foreground));

// keep alpha Channel information
imagesavealpha($canvas, true);

// Output to browser
header('Content-Type: image/png');
imagepng($canvas);

// Destroy resources
imagedestroy($canvas);
imagedestroy($foreground);
?>

5?? Notes

? The file format must support transparency
JPEG does not support transparency, PNG and GIF support.

? Correctly handle alpha blending
Before synthesis, close blending and save alpha after synthesis.

? Check transparency values
The transparency range of imagecolorallocatealpha is 0 (opacity) to 127 (fully transparent) , this is 0 with CSS 1.0 255 Different, pay attention to conversion.